Never thought such a high profile commercial product could ever get open sourced.

You weren't looking around much - Java, Star Office. Arguably also Qt, Blender, Duke3D and id engines, Netscape, Sim City.
And in the OS or DE categories, where CDE is - definitely CP/M, GEM, Symbian, Solaris.

Yeah, open sourcing mostly didn't really help the last four and/or it was done when they were long past their time. Still, kinda neat with CDE, and should be useful at least in ~legacy scenarios.